Many famous comets originally formed in
other solar systems

For immediate release

Boulder, Colo. - June 10, 2010 - Many of
the most well known comets, including Halley, Hale-Bopp and, most recently,
McNaught, may have been born in orbit around other stars, according to a new
theory by an international team of astronomers led by a scientist from the Southwest Research Institute® (SwRI®)
in Boulder, Colo.

Dr. Hal Levison (SwRI), Dr. Martin
Duncan (Queen's University, Kingston, Canada), Dr. Ramon Brasser (Observatoire
de la Côte d'Azur, France) and Dr. David Kaufmann (SwRI) used computer
simulations to show that the Sun may have captured small icy bodies from its
sibling stars while it was in its birth star cluster, thereby creating a
reservoir for observed comets.

While the Sun currently has no companion
stars, it is believed to have formed in a cluster containing hundreds of closely
packed stars that were embedded in a dense cloud of gas. During this time, each
star formed a large number of small icy bodies (comets) in a disk from which
planets formed. Most of these comets were gravitationally slung out of these
prenatal planetary systems by the newly forming giant planets, becoming tiny,
free-floating members of the cluster.

The Sun's cluster came to a violent end,
however, when its gas was blown out by the hottest young stars. These new models
show that the Sun then gravitationally captured a large cloud of comets as the
cluster dispersed.

"When it was young, the Sun shared a lot
of spit with its siblings, and we can see that stuff today," says lead author
Levison.

"The process of capture is surprisingly
efficient and leads to the exciting possibility that the cloud contains a
potpourri that samples material from a large number of stellar siblings of the
Sun," says co-author Duncan.

Evidence for the team's scenario comes
from the roughly spherical cloud of comets, known as the Oort cloud, that
surrounds the Sun, extending halfway to the nearest star. It has been commonly
assumed this cloud formed from the Sun's proto-planetary disk. However, because
detailed models show that comets from the solar system produce a much more
anemic cloud than observed, another source is required.

Levison says, "If we assume that the
Sun's observed proto-planetary disk can be used to estimate the indigenous
population of the Oort cloud, we can conclude that more than 90 percent of the
observed Oort cloud comets have an extra-solar origin."

"The formation of the Oort cloud has
been a mystery for over 60 years and our work likely solves this long-standing
problem," says Brasser.

The article, "Capture of the Sun's Oort
Cloud from Stars in its Birth Cluster," by Levison, Duncan, Brasser and
Kaufmann, was published in the June 10 issue of Science Express.

Funding for this research was provided
by NASA's Astrobiology Institute, Outer Planets Research and Origins of Solar
Systems programs, the Natural Science and Engineering Research Council of
Canada, and Germany's Helmholtz Alliance.